Auto merge of #18514 - asajeffrey:layout-dont-panic-if-no-iframe-bc, r=emilio

Remove sources of panic when laying out an iframe without a nested browsing context

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At the moment, layout panics if it discovers an iframe without a nested browsing context. Under normal circumstances, this is reasonable, but it requires very tight synchronization between script, layout, the constellation and the compositor. In particular, if a layout is in progress when an iframe's browsing context is discarded, this can trigger panic.

This PR fixes this in two ways:

1. Making the pipeline and browsing context ids optional in layout's representation of an iframe.
2. Shutting down layout before discarding a browsing context, rather than after.

This is belt and braces.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #17482 and #18477
- [X] These changes do not require tests because the PR is fixing a panic caused by a race condition

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